The invention relates to an actuator which is equipped with a multistage planetary gear mechanism and has an output shaft before its last planet carrier.
Such an actuator with an electromotively drivable gear mechanism for activating a parking brake is known from EP 1 364 850 A1, with a belt transmission between a motor and a torque converter. The latter is preferably configured as a multistage planetary gear mechanism with an output shaft which is arranged at the last planet carrier. The planet carriers rotate in an internal toothing of the hollow cylindrical gear mechanism housing. Such a gear mechanism provides the increase in torque which is to be aimed at for such an actuator, by virtue of a large step down ratio for the rotational speed in order to be able to use small, high speed electric motors with a correspondingly low output torque. The transmission housing of the previously known actuator is rotatably mounted counter to a restoring spring force so that the instantaneously transmitted torque can be measured by measuring means using the current rotational angle setting. Said publication with a priority date earlier than that of the present document does not contain anything about structural considerations regarding the mounting of the gear mechanism, especially in order to overcome the stressing of materials which increases with the torque.
JP 2 001 173 773 A discloses a planetary gear mechanism which is cascaded axially in multiple stages and is connected directly downstream of a drive motor, whose pinion functions as its first sunwheel on the motor output shaft which projects into the gear mechanism in a cantilevered fashion. The planet wheels of the successive gear mechanism stages each roll in the obliquely toothed inner casing surface of the hollow cylindrical gear box housing which has a constant internal diameter over its length. On the respective planet carrier, a pinion serves as a sunwheel for the planet wheels of the next gear mechanism stage, the last of which is equipped with a stub axle as an output shaft which is radially mounted in the end wall of the gear mechanism housing and projects from it. On said output shaft, a pin which projects into the housing in a cantilevered fashion is attached coaxially in a rotationally fixed fashion in a backwards direction, said pin ending at an axial distance before the motor shaft with its first sunwheel. The planet carriers with their sunwheels rest in a freely rotatable fashion on said pin. So that said sunwheels do not run up against one another axially due to the load, the pin is equipped in each case with an axial stop as a spacer element in a nonmoveable and nonrotatable fashion between the front end of a sunwheel and the planet carrier which is axially adjacent as the next gear mechanism stage. This makes mounting such a gear mechanism extremely difficult. Under load the mechanical stressing of each central bearing pin which is secured only in the output shaft becomes so critical that in the surroundings of the front end of the pin which protrudes freely in the opposite direction it is no longer ensured that the planet wheels will engage in the inner toothing of the housing. For this reason alone such a structure is not satisfactory for the load occurring during operation, with its torque which rises in the longitudinal direction of the hollow cylindrical gear mechanism housing as a result of the central pin being mounted only on one side; even if said structure corresponds to the structural principle of avoiding an over determined bearing.
Actuators of a similar generic type with belt transmission between the motor and gear mechanism are known for a hydraulic parking brake or a mechanical parking brake from DE 1 97 32 168 C1, and corresponding U.S. Pat. No. 6,394,235 B1, both of which are incorporated by reference herein in entirety, or from DE 1 97 48 318 C1, and corresponding U.S. Pat. No. 6,431,330 B1, both of which are incorporated by reference herein in entirety. Harmonic drives or squash plate mechanisms are provided as gear mechanisms with an extremely large step down ratio and a self locking effect. The kinetics of this increase in torque owing to a very high rotational speed gearing reduction ratio in turn requires materials to be used which can be loaded to a very high degree and are therefore costly.
However, using high strength special materials to cope with the forces occurring in the gear mechanism entails costs which are hardly compatible with the cost pressures placed on suppliers in the motor vehicle industry.